I've installed a BES in our network and everything worked fine until recently. I've been able to activate our employees Blackberry through the BES, but I just noticed that when I try and activate any users now it will not work, the Blackberry phone that I'm trying to activate just sits there and finally gives me a message saying "the server cannot be found or is not responding" , but all the phones that I activated before are still working.

So, does anybody know why I'm unable to activate any new phones on the BES, but all the phones I activated when it was working are still functioning???

Call the wireless carrier and verify that the lines have an ENTERPRISE BlackBerry Data Plan provisioned properly on the line.
If it's only a PERSONAL BlackBerry Data Plan, then you won't be able to activate on BES.